‘TEMPERATURE 65 
gravel soils. Repeated tests of this have been made by: simultaneous read- 
ings in permanent and newly made holes, and the results have always. been 
the same. It has even-been found that the error is usually less than 1 degree 
when the hole is left uncovered, if it is more than 9 inches deep. A slight 
source of error lies in the fact that the thermometer must be raised.to make 
the reading. With a little practice, however, the top of the 
column of mercury may be raised to the surface and read be-. 
fore the change of temperature can react upon it. This is 
especially important in very moist or wet soils where the bulb 
becomes coated with a film of moisture. This evaporates 
when the bulb is brought into the air, and after a moment or 
two the mercury slowly falls. 7 
Regular soil thermometers are indispensable when read- 
ings are desired at depths greater than 12-18 inches. They 
possess several disadvantages which restrict their use almost 
wholly to permanent stations. It is scarcely possible to carry 
them on field trips, and the time required to place them in 
the soil renders them practically useless for single readings. 
Moreover, the instruments are expensive, ranging in price 
from $7 for the two-foot thermometer, to $19 for the eight- 
foot one. When it is recognized that deep-seated tempera- 
tures are extremely constant and that the slight fluctuations 
affect, as a rule, only the relatively stable shrubs and trees, 
it is evident that such temperatures are of restricted impor- 
tance. Still, in any habitat, they must be ascertained before 
they can well be ignored, though it is unwise to spend much 
time and energy in their determination. Soil thermometers 
of the form illustrated may be obtained from H. J. Green, 
Brooklyn, 
96. Maximum-minimum thermometers. These are used 
for determining the range of temperature within a given 
period, usually a dav. Since they are much cheaper than 
thermographs, they can replace these in part, although they 
merely indicate the maximum and minimum temperatures for 
the day, and do not register the time when each occurs. The 
maximum is a mercurial thermometer with a constriction in the tube just 
above the bulb; this allows the mercury to pass out as it expands, but 
prevents it from running back, thus registering the maximum temperature. 
The minimum thermometer contains alcohol. The column carries a tiny 
dumbbell-shaped marker which moves down with it, but will not rise as 
Fig. 16. Soil 
thermometer 
